There are strong indications that the global mean temperature is rising. The increase in temperature is mainly attributed to the increase in the atmospheric CO2 concentration, [CO2], the result of anthropogenic emissions from burning fossil fuels and changes in land-use, mainly deforestation. The total emissions of CO2 cannot be accounted for by the measured increase in [CO2], nor by the amounts estimated to be taken up by oceans and terrestrial ecosystems, hence there is a missing sink for CO2, where the boreal forest may play an important role. The tree growth in boreal forests is limited by climate and nutrients, hence could a warmer climate, and more available nutrients, increase production. Increased temperature may, however, also accelerate decomposition of soil organic matter. Since production and decomposition is affected by the availability of nutrients, increased N-deposition or forest fertilisation may mitigate the effects of global warming. The main objective of the present study was to determine the effects of nutrient supply on soil respiratory components in a boreal Norway spruce forest, especially the effects on heterotrophic and rhizospheric activity. To partition soil respiration into these components, tree-girdling, which terminates the carbohydrate supply from photosynthesis to the rhizospheric component, was used. The study was conducted in 40-year-old Norway spruce stands in Northern Sweden, which were growing at growth-limiting and optimised availability of soil nutrients, respectively. During the period of annual maxima of soil respiration, fertilisation decreased rhizospheric soil CO2 efflux by approximately 50%. Also heterotrophic soilCO2 efflux was approximately 40% lower in fertilised plots, than in non-fertilised plots, despite a three-fold higher above-groundproduction in thefertilised plots. In conclusion forest fertilisation contributes to C sequestration by increasing biomass production as well as retarding decomposition of soil organic matter.
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